1. Field of the Invention
The present invention relates to a control method for compensating for load disturbances by systematically controlling a plurality of types of distributed power sources having different load-following capabilities for the load disturbances.
Priority is claimed on Japanese Patent Application No. 2009-204071, filed Sep. 3, 2009, the content of which is incorporated herein by reference.
2. Description of Related Art
Recently, power sources using solar photovoltaic or wind-power generation are being introduced and promoted as a national strategy. However, since power sources using natural energy such as solar photovoltaic or wind-power generation depend significantly on weather, there is concern that supply reliability may not be guaranteed, and supply-and-demand balance may be difficult to obtain in a commercial electricity system.
To address this problem, there is a microgrid which alleviates the load on a commercial electricity system and provides an interworking relationship by generating electricity to respond to demand within a certain area using distributed power sources capable of adjusting outputs (this is called a load-following operation). The energy supply system (hereinafter, referred to as a microgrid) using distributed power sources constructed by employing the microgrid concept to perform a load-following operation is advantageous in that: (1) it is possible to prevent the natural energy power sources having an unstable output from harming the commercial electricity system; and (2) in the event of abnormality in the commercial electricity system, such as interruption of an electricity service, it is possible to keep an autonomous operation with respect to the load within a corresponding area with a stable electricity service quality (frequencies or voltages) by disconnecting the microgrid from the commercial electricity system.
For example, Japanese Unexamined Patent Application, First Publication No. 2006-246584 (hereinafter referred to as Patent Document 1) discloses a method for performing the load-following operation of the distributed power source in the microgrid. In this control method, using a plurality of types of distributed power sources having different load-following capabilities, the load disturbance compensation is realized by setting, in advance, a frequency bandwidth of the load disturbance to be compensated for depending on the load-following capability of the distributed power source and controlling the output of the distributed power source to follow the load disturbance preferentially from the power source having a lower load-following capability.
The load disturbance compensation operation described in Patent Document 1 will be described with reference to FIG. 5. FIG. 5 is a control block diagram when output control of three power sources is performed. In this control method, the disturbance component of a measured load electric power PLOAD to be compensated for in a gas engine generator 1, amplitude of which is limited by an amplitude limiter 41, and further a predetermined frequency component of which is extracted by a LPS (low pass filter) 42, is set as an output specification value PsG to the gas engine generator 1. The gas engine generator 1 outputs an electric power P1 based on this output specification value. Moreover, a load disturbance that could not be followed using the gas engine generator 1, which is obtained by subtracting the output an electric power P1 of the gas engine generator 1 from the load electric power PLOAD, frequency of which is limited by an amplitude limiter 43, and further a predetermined frequency component of which is extracted by a LPF 44, is set as an output specification value PsBES of a nickel metal hydride battery 2. The nickel metal hydride battery 2 outputs an electric power P2 based on this output specification value. Furthermore, a load disturbance that could not be followed using the gas engine generator 1 and the nickel metal hydride battery 2, which is obtained by subtracting the output powers P1 and P2 of the gas engine generator 1 and the nickel metal hydride battery 2 from the load electric power PLOAD, frequency of which is limited by an amplitude limiter 45, and further a predetermined frequency component of which is extracted by a LPF 46, is set as an output specification value PsEDLC of an electric double-layer capacitor 3. The electric double-layer capacitor 3 outputs an electric power P3 based on this output specification value. Through these control operations, the load disturbance compensation can be performed.
According to this control method of Patent Document 1, considering the load-following capabilities of the distributed power sources, an electricity storage device such as the secondary battery or the electric double-layer capacitor is usually used as a power source which compensates for the disturbance in the fastest frequency bandwidth. Since the chargeable/dischargeable capacity of the electricity storage device depends on the remaining capacity in the capacitor body (state of charge; hereinafter, referred to as SOC), if SOC management is not performed, it may be impossible to obtain a satisfactory output for the demanded output specification as the SOC increases or decreases. In this regard, the SOC management control is necessary to effectively implement the load-following operation in the electricity storage device having a storage capacity as small as possible. As a method for implementing the SOC management control, for example, using the electric double-layer capacitor, Japanese Unexamined Patent Application, First Publication No. 2007-020361(hereinafter referred to as Patent Document 2) discloses a method for subtracting the difference between a setup value and a detection value of a direct-current from an output specification through a voltage controller configured by such as a PI controller or a PID controller.
However, even if the method of Patent Document 2 is used, since the electricity storage device autonomously adjusts the output of itself to perform the SOC management, the precision of the load-following operation may be degraded if means for compensating for the output fluctuation associated with the SOC management is not provided.